The present invention relates to a non-sealable, biaxially oriented, transparent multi-layer film. The invention also relates to a process for the production of this film and to its use.
The hitherto known non-sealable, biaxially oriented multi-layer films of propylene polymers, formed of a base layer and one or two covering layers have, in particular, the disadvantage that they are unsatisfactory in view of their slip and anti-blocking characteristics, while they simultaneously have good antistatic properties and a high transparency. In addition, the scratch resistance of these films, which is another important property, is more or less insufficient.
Many attempts have already been made to obtain polypropylene multi-layer films with improved properties by incorporating additives into the covering layers, with a view to producing the desired properties. For example, it has previously been suggested to include an effective amount of a thermoplastic resin having a melting point above the melting point of the polypropylene (incompatible thermoplastic resin) or of an inorganic pigment into the covering layers. The inorganic pigments mentioned comprise aluminum oxide, silicon dioxide, titanium dioxide, a synthetic zeolite and the like, and also micro glass beads. However, all these films are unable to meet the actual requirements.
U.S. Pat. No. 4,419,411 describes a polypropylene multi-layer film, the base layer of which comprises a polypropylene of high stereoregularity and the skin layer of which comprises a polyolefin of comparatively low stereoregularity and additionally contains a surface-modifying agent, for example, a silicate or a silicone oil. The multi-layer film is stated to have a low coefficient of friction which remains constant over a wide temperature range.
European Patent Application No. 0 138 203 teaches that, instead of adding anti-blocking and slip agents and optionally further additives as individual components to a polyolefin material, it is economical to form a combined concentrate from these components prior to the admixing operation and to add this concentrate to the polyolefin material in one batch.
In European Patent Application No. 0 124 310, a non-sealable, biaxially oriented multi-layer film of propylene polymers is described, which is stated to have good slip and anti-blocking properties and, at the same time, a high transparency. This film comprises a base layer and at least one covering layer, the covering layers containing, as an additive for the improvement of properties, a finely divided inorganic pigment, for example, silicon dioxide, aluminum silicate, sodium-aluminum silicate and/or carbon black, in an amount such that the biaxially drawn multi-layer film no longer shows any turbidity worth mentioning. The average particle size of the pigment is indicated as ranging from 0.2 to 5 .mu.m. The shape (form) of the pigment particles is not specified; it is obviously considered inessential.
The above-described polypropylene multi-layer film lacks, in particular, sufficient antistatic properties and scratch resistance and its slip and anti-blocking values still leave much to be desired. Just as all the other presently known non-sealable, oriented multi-layer films of propylene polymers, this film cannot be subjected to a corona treatment on both surfaces, since by this treatment the anti-blocking behavior is reduced to such an extent that blocking (sticking) of the film on the film roll occurs. Frequently, such sticking of superimposed film surfaces not only gives rise to elongation and tearing of the film, but can even prevent unwinding from the roll.
As is evident from the cited art, the additives for the improvement of properties are, in each case, combined in a covering layer and/or base layer. The effects which the individual additives within a layer have upon each other are therefore not considered as being decisive for the characteristic features of the resulting film.
Moreover, sealable, biaxially oriented multi-layer films of propylene polymers formed of two or three layers have also been disclosed, which contain one or several additives in their sealing layers to obtain good slip properties and, in particular, good machine runability in high-speed packaging machines, which is the most important property. These films, which are not the subject matter of the present invention, do not only constitute a specific group, they are also intended for purposes which are completely different from those for which the non-sealable, oriented multi-layer films are used.